Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : The free histidine is a dominant buffering system at physiological pH than the protein-bound or dipeptide histidine.
Reason (R) : In proteins or dipeptides, the imidazole pKa decreases substantially-in combination with other amino acids- to a level which is more than 1 pH unit removed from intracellular pH.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. Both (A) and (R) are false.

Hint:

Free histidine has a low concentration and a pKa of 6.0. In dipeptides like carnosine, the pKa shifts upward to 6.83, making it a highly effective physiological buffer near cellular pH.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Histidine contains an imidazole side chain.
The pKa of this imidazole group is approximately 6.0 in free histidine, which allows it to act as a physiological buffer in biological systems.

Step 2: Detailed Explanation:
Let us analyze both statements:
- Assertion (A) is false: Within cells, the concentration of free histidine is extremely low.
Instead, it is the protein-bound histidine residues and histidine-containing dipeptides (such as carnosine and anserine) that are highly abundant in tissues.
These protein-bound and dipeptide forms serve as the primary and dominant intracellular buffering systems at physiological pH, not free histidine.
- Reason (R) is false: In proteins and dipeptides, the local chemical environment shifts the pKa of the imidazole group.
This shift is generally upward (increasing to around 6.8–7.0) rather than downward.
For example, in the dipeptide carnosine (beta-alanyl-L-histidine), the pKa of the imidazole ring is 6.83.
This shift brings the pKa closer to the physiological intracellular pH ($\approx 7.2$), which significantly enhances its physiological buffering capacity.
It is incorrect to state that the pKa decreases to a level more than 1 pH unit away from physiological pH.
Therefore, both Assertion (A) and Reason (R) are false, making Option (D) the correct choice.

Step 3: Final Answer:
Both Assertion (A) and Reason (R) are false.